The field of art to which this invention pertains is polycyanate esters of polyhydric phenols.
Polycyanate esters of polyhydric phenols, as described in U.S. Pat. No. 3,553,244, are made by reacting cyanogen halide with polyhydric phenols. These esters will form thermoset compositions by cyclotrimerization of the cyanate groups to polycyanurate networks. Such thermoset compositions are finding numerous uses in a wide variety of industrial applications. However, there is a continuing effort to widen the uses for these compositions and to improve their properties, such as toughness.
The use of synthetic rubbers to toughen thermosetting resins is a well developed procedure. Functional oligomeric elastomers when blended with thermosettable resins will phase separate during the curing reaction and will be covalently bonded at the interface of the phases. The low modulus of the elastomer rich domains reduces the stiffness of structural composite matrices below levels required for performance at 180.degree. F. or above, particularly when tested in the moisture saturated condition. Dimensional stability, especially critical for multilayer laminates used as electrical circuitry for computers, is also unsatisfactory when rubber modification exceeds 5 to 10 percent.
In U.S. Pat. Nos. 4,157,360 and 4,334,045, polycyanate esters are modified by blending with high levels of polyestercarbonates. Such blends are hard and non-conforming in the uncured state and require high temperatures for gelation due to the difficulty of incorporating active curing catalysts in the blends. Structural composites and film adhesives do not possess the tack and drape required for conventional lay-up on mold or part surfaces. The cured blends rarely develop phase-separated morphologies.
In U.S. Pat. No. 4,094,852, blends of polycyanate esters and high boiling plasticizers are described. Among the plasticizers disclosed are carbonic acid esters and carboxylic acid esters having molecular weights up to 10,000.When cured, phase-separated morphologies do not develop.
Blends of polycyanate esters and acrylonitrile-butadiene copolymers are disclosed in U.S. Pat. No. 3,649,714. In these blends, both the polycyanate ester and the acrylonitrile-butadiene copolymer cure when heated.
British Pat. No. 1,055,524 states that polycyanate esters can be modified with a number of polymers possessing a Tg of less than 100.degree. C., such as homo and copolymers of vinyl chloride, polyvinylidene chloride, polystyrene, styrene-acrylonitrile copolymers, butadiene-acrylonitrile copolymers, natural and synthetic rubber, ethylene-vinyl acetate copolymers and the like. When hardened, the polycyanate products of the invention give rise to highly cross-linked, glass-clear synthetic resins with hard surfaces. The development of phase-separated morphologies is not taught by the patent.
In U.S. Pat. No. 4,631,319, polycyanate ester resins are modified with hydroxyl terminated aliphatic and cycloaliphatic polyester resins. Such modification with non-aromatic polyesters results in compositions which do not retain the stiffness under hot-wet conditioning necessary to meet matrix requirements for aircraft structural composites. Such compositions are also susceptible to destructive hydrolysis under conditions of prolonged moisture conditioning, such as 14 days immersion in 160.degree. F. water or 48 hours in boiling water.
Polyepoxide resins have been modified with thermoplastic polymers as described in U.S. Pat. Nos. 4,661,559 and 4,567,216. Polysulfones, polyarylethers, phenoxy resins and the like are incorporated with the epoxy resin and diamine curing agent. Epoxy resins even when so modified do not possess the 250.degree. C. Tg characteristics of polycyanurates nor their low moisture absorption. Epoxy resins are unsatisfactory for some higher temperature, moisture sensitive applications, such as composites and adhesives for supersonic aircraft.
Stiff, tough thermoset composites based on epoxy resins are described in U.S. Pat. Nos. 4,656,207 and 4,656,208. Oligomeric amine terminated aromatic resins in the amount of at least 35 weight percent are blended with epoxy resins. When cured, the aromatic resins form a discontinuous glassy phase. In addition to the aforementioned limitations of epoxy resin matrices, these composites lack tack, drape and out-life characteristics sought in structural composites and adhesives due to the high concentration of aromatic resins.